This invention provides a means for testing large sized transmissions. In particular the invention is useful in testing transmissions which couple a pair of turbine engines to the rotor of a helicopter. A typical helicopter transmission will have two input shafts. Each input will connect to the shaft output of a turbine engine. Under full load conditions the input shafts will rotate at about 12,000 rpm. Typically, the transmission will have two outputs. One output will be from a shaft which rotates at perhaps 3000 rpm and is used to drive the tail rotor. The main lift rotor of the helicopter is driven from a transmission output shaft which rotates at about 190 rpm. Total power fed to a transmission can be over 3,000 Hp. The transmission and gearing contained therein must be sized to handle this power level. Consequently many transmissions have an overall volume that is in excess of 25 cu. ft.
A transmission has to be tested before it can be incorporated into the design of an aircraft. Tests include checking such things as reliability, life expectancy and efficiency. The life cycle tests may run as long as 2,000 hours. Testing was originally done by configuring a test stand so that the transmission was driven by the same engines that would be used in flight. Operating these engines for long periods of time not only wore them out but also created a noise problem for workers in the vicinity. The engine driven test set up was also costly to operate due to the fact that the energy coming out of the transmission had to be dissipated into something. Many test set ups used a water brake. Expending several thousand horsepower for hundreds of hours is a costly undertaking.
More recently, a closed loop mechanical system has been used for testing transmissions. With this approach an electric drive motor is shaft coupled to the input of the transmission. To assure rated speed at the transmission input shaft, the coupling from the drive motor may include use of a speed changing gear box. The output shaft of the transmission is coupled to a right angle gear box. By right angle gear box is meant that the output shaft of the gear box is at a 90 degree angle with respect to the input. Using three such right angle gear boxes coupled one to the next by appropriate lengths of shafting will bring the output shaft of the third gear box pointing at the input shaft coupling the drive motor to the transmission. By placing a differential gear case between the drive motor and the input to the transmission, the output from the third right angle gear box can be coupled back into the input drive of the transmission. Placement of a rotating differential gear box between the second and third right angle gear boxes completes the test set up. An external drag force applied to the rotating cage member of this differential gear box serves to load up the transmission whenever the drive motor operates. This closed loop system is commonly known as a four-square test set up with dynamic torque applied by a differential gear train.
Other mechanical four-square test set ups have been configured. All of the prior art systems have certain deficiencies. With some a power dynamometer or variable speed brake is required to absorb the output power of the transmission. In the four-square mechanical feedback loop systems, the mechanical resonances of the closed loop system can cause an inordinate amount of wear on the gears within the system. When two similar transmissions are arranged in a loop configuration, the resulting test set-up becomes very complex.